Prostaglandins are eicosanoids derived from arachidonic acid (20-carbon polyunsaturated fatty acid). They act as paracrine and autocrine signaling molecules.

Saturated fatty acids have higher melting points and are typically solid at room temperature due to tight packing. They are more stable and less prone to oxidation than unsaturated fatty acids.

Cholesterol molecules insert between phospholipids in the bilayer, regulating membrane fluidity. At high temperatures, it reduces fluidity; at low temperatures, it increases it.

HMG-CoA reductase catalyzes the conversion of HMG-CoA to mevalonate and is the rate-limiting step in cholesterol biosynthesis. It is inhibited by statins.

Triglycerides are the primary storage form of lipids in adipose tissue, accounting for >95% of stored lipids. They are mobilized during energy deficit states.

HDL mediates reverse cholesterol transport, removing cholesterol from peripheral tissues and arterial walls via ABCA1 and SR-BI pathways, then transferring it to the liver via CETP-mediated exchange with apoB-containing lipoproteins. Higher HDL is associated with reduced cardiovascular risk.

Cholesterol is the universal precursor for steroid hormone synthesis (cortisol, testosterone, estrogen) and bile acid synthesis. The side-chain cleavage enzyme (P450scc) catalyzes the first committed step of hormone synthesis, while 7-alpha-hydroxylase initiates bile acid synthesis.

Unsaturated fatty acids with cis double bonds introduce kinks in the carbon chain, preventing tight packing and increasing membrane fluidity. Cholesterol and saturated fatty acids decrease fluidity. Temperature and cholesterol content can be modulated to maintain optimal fluidity.

Linoleic acid (omega-6 PUFA) is essential because humans lack the enzyme delta-12 desaturase needed to introduce double bonds beyond carbon-9. Only plant-based sources provide linoleic and alpha-linolenic acids. Other fatty acids can be synthesized de novo.

Phospholipids have both hydrophilic (phosphate group) and hydrophobic (fatty acid chains) regions, making them amphipathic. This property allows them to spontaneously form bilayers in aqueous solutions, serving as the structural foundation of biological membranes.